1. Field of the Invention
This invention relates to thixotropic organosiloxane compositions. More particularly, this invention relates to two or three part moisture curable, high viscosity organosiloxane compositions capable of being applied to vertical surfaces with negligible post-application flow. In the two part system, the catalyst portion of this invention comprising the crosslinking agent, the crosslinking catalyst, the thixotropic agent, and the thixotropic enhancing diluent is packaged separately from the base system portion, i.e., the crosslinkable polyorganosiloxane and the hydroxyl-containing filler. In the three part system, the thixotropic agent and the thixotropic enhancing diluent is packaged separately from the base and the catalyst portions: the catalyst portion being the crosslinking agent, and the crosslinking catalyst.
2. Background of the Art
In order to copy the details of shaped articles, molding composition is first applied to the surface of the object to be copied. After hardening, the composition is removed, retaining a negative copy or mold of the surface or object. Then, when a hardenable material is applied to the negative copy, a copy of the original results.
Although a wide variety of materials have been used for making molds, such as plasters, cements, and natural and synthetic polymeric compositions; for detailed and/or complicated shapes, the preferred material is silicone rubber. These organopolysiloxanes have many favorable properties for preparing negative molds including relatively high tensile strength, high tear strength, and excellent elongation properties. These properties permit silicone rubber formulations to mold complex and detailed shapes seamlessly and to be removed easily from the molded article by stretching.
Unfortunately, silicone rubber compositions are relatively difficult to apply to large vertical surfaces. Conventional silicone molding compositions typically comprise a two or three part formulation and each of these parts must be mixed together just prior to using. Next the silicone is conventionally applied to the surface of the article to be copied, usually by brushing, troweling, or spraying. To ensure that the curable silicone molding composition is in intimate contact with all of the surface features of the article to be copied, it is highly desirable that during the application processing, the initial viscosity of the composition is sufficiently low as to enable the composition to flow around, over and into the details and interstices of the surface to be copied. However, when such a low viscosity composition is applied to a vertical surface, the force of gravity causes the material to flow to such an extent that obtaining a sufficiently thick mold has proven to be exceedingly difficult.
In order to overcome this vertical run-off problem, the prior art created thioxtropic compositions that exhibit the desirable processing characteristics of having low viscosity, when subjected to high shear conditions caused with brushing, troweling or by the action of the pump and nozzle apertures at a spray head, and relatively high viscosity immediately after contact with the surface of the article to be copied; resulting in minimum running or slumping of the hardening silicone composition.
Various materials have been used to impart thixotropy to curable organosiloxane compositions.
U.S. Pat. No. 5,036,131, which issued to A. Himstedt on Jul. 30, 1991 describes silicone dispersions containing 100 parts by weight of a liquid silanol-terminated polydimethylsiloxane, a moisture activated curing agent, and finely ground alumina trihydrate. The thixotropic agent is the combination of an untreated fume silica and a liquid silanol-terminated polydiorganosiloxane containing phenylmethylsiloxane or 3,3,3-trifluoropropylsiloxane units that are incompatible with the polydimethylsiloxane. The polydiorganosiloxane reacts with the silica to form a surface that is incompatible with the polydimethylsiloxane.
Russian patent 1,502,586 issued to A. Chuiko et al. describes protective coatings containing a polymethylphenylsiloxane, toluene and a pyrogenic form of silica that has been surface-modified with methyl groups.
A disadvantage of using the incompatible filler technique described in the patents to Himstedt and Chuiko, et al. in the base portion of a two part moisture curable organosiloxane composition comprising a silane containing alkoxy or other hydrolyzable groups as the curing agent and an organotin compound as the curing catalyst is the difficulty of processing the resultant highly thixotropic material.
Other thixotropic agents that have been used in curable organosiloxane compositions include silica treated with various organosilicone compounds, chalk, dolomite, silicone polymer chains grafted with primary or secondary amine functions, and organosiloxane/oxyalkylene copolymers in combination with a filler containing hydroxyl groups.
To achieve adequate storage stability, some moisture curable organosiloxane compositions, particularly those containing tin compounds as the catalyst for the crosslinking reaction, are packaged with the tin compound and crosslinkable polyorganosiloxane in separate containers. The crosslinking agent is typically packaged together with the catalyst.
Because only a small volume of crosslinking agent relative to the volume of crosslinkable polyorganosiloxane is required to achieve the desired degree of crosslinking, the part of the composition containing the crosslinking agent typically contains a quantity of diluent that will allow this part to be blended with the part containing the crosslinkable polyorganosiloxane in a volume ratio of 1 part of curing agent portion to at least 10 parts of the portion containing the crosslinkable polyorganosiloxane. Preferably, the diluent also acts as a solvent for the crosslinking agent and the catalyst.
For some end use applications of two-part moisture curable organosiloxanes, thixotropy would be undesirable. If the thixotropic agent were in the portion of the composition containing the crosslinkable polyorganosiloxane, typically referred to as the “base” portion, a user who wished to alternate between thixotropic and non-thioxtropic compositions would have to change the container of the base. If the thixotropic agent were in the crosslinking agent portion, using the conventional volume ratio of base to crosslinking agent portion of 10:1, the volume of material to be transferred during the change from a thixotropic to a non-thixotropic composition would be 1/10 of the volume to be transferred if the thixotropic agent were in the base portion.
Many diluents, however, used in the crosslinking agent portion of conventional two part moisture curable organosiloxane compositions will not dissolve the organosiloxane/oxyalkylene copolymers often used as thixotropic agents in these compositions. The resultant two-phase composition results in non-uniform curing of the composition.
Donatelli, et al. in U.S. Pat. No. 5,563,210 disclose that certain organosiloxane oligomers containing phenyl alkyl siloxane units as at least a portion of the repeating units are suitable diluents and solubilizers for the organosiloxane/oxyalkylene copolymers used as thixotropic agents for many moisture curable organosiloxane compositions. Thus, when these curing agent compositions are mixed with the base crosslinkable polyorganosiloxane composition, a homogeneous curable composition is formed.
Those skilled in the art are continually searching for more effective, less costly diluent/solubilizers for the thixotropic containing, curing agent compositions.
It is well known in the art that for some fillers, particularly reinforcing fillers such as silica, treatment of the filler to react, i.e., eliminate a portion of the hydroxyl groups present on the surface of the filler particles is often necessary to prevent a phenomenon referred to in the art as “creping” or “crepe hardening”.
This is especially true with precipitated silicas. If the hydroxyl groups are left on the silica filler in the base, the composition thickens to a point wherein it becomes unworkable. Therefore, the hydroxy radicals are removed to stabilize the composition.
In view of the above, the present inventors have surprisingly discovered that, with the use of C2–C24 aliphatic diols, preferably C2–C12 aliphatic diols, and most preferably 1, 2 propane diol in lieu of the heretofore art recognized diluent additives; a most effective diluent solubilizer can be realized. Serendipitously, it has been discovered that the diols of this invention significantly enhance the thixotropic characteristics of the curable silicone composition.